It looks like this might be an invalid issue after all. (Valid) HTML tags can't have whitespaces after the < character. Although it's somewhat interesting that FILTER_SANITIZE_STRING is a little bit more stricter.
There was another code in play in the original PHP application I was looking at that was fixing the formatting of the resulting string after the strip_tags call. Please feel free to close this ticket as invalid, and sorry for the false positive.

> (Valid) HTML tags can't have whitespaces after the < character.
That.
Anyhow, strip_tags() is not the appropriate way to eliminate XSS
vulnerabilites[1]. This should be documented in the manual.
[1] <http://news.php.net/php.internals/102462>